1) How does the energy enter?
2) From where can the energy leave? Convection, conduction, radiation.
3) The First Law of thermodynamics requires emissivity exactly equal absorptivity at any given wavelength. The box should be dreadfully black in the visible and dreadfully white in the IR.
4) Look up "hot mirrors" and "cold mirrors."

You now know the optimum relative dimensions and the best materials of construction.